Why do you need a locker in an RV?
Anything other than rebuilding what you have now, or finding a used one to bolt in with same or different gearing, will be much more than a simple upgrade.
I have a G20 RV as well. The right 14 bolt will fit but hard to find in a 3.40. It’s not the transmission that will matter but where the shock mounts are on the axle and how far the spring perches are apart on that specific axle. Is yours primarily towing something heavy? Off roading? Hard to say if you’d need a locker....